The di-substituted bismuth oxide based oxide ion conducting electrolyte system Bi-2.5 (+) xPb0.5YO5.75 (+) (3x / 2 -6) (X = 0, 1 and 2) has been investigated by X-ray powder diffraction, a.c. impedance spectroscopy, thermal analysis and X-ray photoelectron spectroscopy. delta-Bi2O3 type phases are observed for all compositions studied and showed no obvious changes in structure up to 850 degrees C. XPS results show an increasing trend in binding energy with increasing x-value. Results from thermal analysis and the thermal variation of the lattice parameter indicate a small degree of reduction occurs at high temperature, which is preserved on quenching. This reduction is reversed on heating at intermediate temperatures. The redox reactions appear to be correlated to a degree of curvature in the Arrhenius plot of conductivity. Measurement of transference numbers indicates that these materials are predominantly ionic conductors, with conductivity values at high temperature comparable with mono-substituted analogues. (C) 2013 Elsevier B.V. All rights reserved.